1. Field of the Invention
The present invention pertains generally to lithography, and more specifically to lithography methods using short wavelength electromagnetic radiation.
2. Description of the Background Art
Lithography has been the key enabling technology for the steady performance improvement in semiconductor integrated circuit (IC) devices over the last thirty years. By reducing the feature size, the density of components as well as the speed and functionality of microchips have doubled every two to three years. The current generation of lithography methods employ visible or ultraviolet light and refractive lens objectives to image the mask pattern onto a pattern-forming resist layer on a wafer and subsequently develop the latent image to complete a pattern transfer process. Since the printable feature size is limited by the wavelength of the light, ever decreasing wavelengths are being used for lithography applications. Further reduction in wavelength using current refractive lens-based lithography methods can be extremely difficult and is limited by a lack of suitable materials for making the imaging lenses.
This difficulty was recognized more than ten (10) years ago and alternative lithography methods (collectively known as the next-generation lithography methods) are currently being developed. The next-generation lithography (NGL) methods employ either charged particle beams, e.g., electron and ion, or electromagnetic radiations having wavelengths substantially shorter than one-hundred and fifty-seven nanometers (157 nm)—the shortest wavelength to be used in the current generation lithography tools. At various times, next-generation lithography methods have included proximity X-ray lithography, ion projection lithography, extreme ultraviolet lithography (EUVL), and electron beam projection lithography (EPL). Presently, EUVL and EPL are considered the most promising candidates.
The EUVL method employs extreme ultraviolet radiation having a wavelength of approximately thirteen nanometers (13 nm). A EUVL lithography camera typically consists of 4 to 6 aspherical multilayer mirrors arranged at near-normal incidence, which require a demanding multilayer coating (often with a gradation in multilayer period across the optic) on large diameter aspherical optical mirror surfaces with figure control at almost the atomic level. The camera is not axially symmetric but has a ring-shaped printing field. To produce an illumination field matching that of the printing field of the camera, a condenser consisting of a large number of multilayer mirrors is required. For example, the Engineering Test Stand (ETS) developed by the EUVL LLC and the virtual national laboratory has ˜20 multilayer mirrors in the condenser and has 4 multilayer mirrors in the camera. Radiation arriving at the photoresist experiences nine reflections from its origin at the source. The number of multilayer mirrors and thus the number of reflections in the camera may increase by two in EUVL cameras designed to achieve resolution better than 50 nm. Because of the large number of multilayer mirrors must be used, only a small percentage of the EUV source power is delivered to the wafer, while a large amount flare is added to the exposure. The large number of multilayer mirrors also imposes stringent requirement on the precise matching of the multilayer spacings, alignment, mechanical and vibrational stability. Consequently the costs of initial tool, replacement, and maintenance are extremely high.
Camera designs based on optics developed for x-ray microscopy have also been proposed. Transmission objectives developed for x-ray focusing and imaging applications include zone plates and compound refractive lenses. A compound refractive lens consists of many spherical or parabolic shaped lenses aligned along the optical axis. A large number of individual lenses are needed to obtain a short effective focal length because the focal length is inversely proportional to the number of lenses, which typically have a focal length of tens to hundreds of meters. Therefore, compound refractive lenses are not likely to be useful as objectives in the next generation lithography methods because they do not have the required numerical aperture with an acceptable system throughput.
Zone plates consist of concentric rings with alternative materials. The positions of the rings are determined by a simple equation and the ring width decreases with increasing radius. They are currently the highest resolution transmissive optic, demonstrating a resolution of better than 25 nm in the 2-5 nm spectral region. The focal length of a zone plate lens is inversely proportional to wavelength and therefore the zone plate is highly chromatic. This chromaticity precludes its application in lithographic imaging cameras since an illumination beam with an extremely narrow spectral bandwidth would be required to limit chromatic aberrations should a printing field of sufficient size be required. This would consequently severely limit the energy from the source that can be used for exposing the photoresist.